In the brain, glucose serves as the main source of energy if not the one and only.  If patients with brain cancer can stop ingestion of carbohydrates/sugars, the disease could be controlled to some degree.  That is why ketogenic diet has been tested to treat patients with brain cancer even though the efficacy remains largely unknown.

But glucose is not the only energy source for brain cancer.  A study released in 2014 in the journal Cell suggests that glioblastoma and brain metastases can use acetate as an energy source to support their growth and progression.

Acetate is found in vinegar. Does that mean patients with glioblastoma and brain metastases should not use vinegar?  Vinegar is often used as a taste adjuster, and often in small amounts.  But this study apparently suggests that patients with this type of cancer would be better off having zero intake of vinegar.

Keep in mind that dietary acetate or vinegar may not be the main source of acetate in the blood.  Acetate in the blood can be derived from other substrates such as alcohol.  In any case, the blood always carries some acetate that the brain cancer can use.   That is, it can be very hard or impossible to completely prevent the formation of acetate in the blood.

The major sources of energy for brain cancer like glioblastoma are glucose and glutamine.  And also remember that glucose in glioblastoma does not only go through the anaerobic glycolysis to generate low energy and lots of lactic acid, but also the citric acid cycle (CAC) to produce large amounts of energy and metabolites for biosynthesis.  

Acetate enters the CAC through which it produces energy and other metabolites just like pyruvate - ab intermediate compound generated through the glycolytic process from glucose.  So acetate is not the only problem for glioblastoma.  (David Liu)